A C1ListViewControlGroupItem set to 'checkboxlist' will create a checkbox multi-select item.

The markup used to create a checkbox C1ListViewControlGroupItem will resemble the following:

<cc1:C1ListViewControlGroupItem ControlGroupType="checkboxlist" LabelText="Priority">
    <innerlistcontrols rows="1" width="400px">
       <asp:ListItem>1</asp:ListItem>
       <asp:ListItem>2</asp:ListItem>
       <asp:ListItem Selected="True">3</asp:ListItem>
       <asp:ListItem>4</asp:ListItem>
       <asp:ListItem>5</asp:ListItem>
     </innerlistcontrols>
 </cc1:C1ListViewControlGroupItem>